Increasing the nonlinear conversion efficiency of metals at the nanoscale often relies on the use of plasmonic near-field hot spots in metallic nanostructures with tiny gaps. However, such nanoscale gaps often quench the second-harmonic generation (SHG) due to the cancellation of surface dipoles, making it a challenge for efficient optical frequency-conversion and nonlinear nanoscopy enhancement at the deep-subwavelength scale. Here we demonstrate that the SHG silencing can be overcome in the sub-nanometer gap of a single plasmonic particle-on-film nanocavity, which exhibits a surprisingly larger SHG conversion efficiency up to 1.0×10^(-8) W-1. On the one hand, laser scanning confocal imaging of single particle-on-film nanostructures reveals unambiguously that the enhanced nonlinear optical emissions originate from resonant excitation of the gap plasmon, which amplifies the nonlinear source at the fundamental frequency. On the other hand, full-wave electromagnetic simulations uncover that the structural symmetry breaking in the plano-concave gap suppresses the cancellation of SH dipoles, which, together with the resonance excitation enhancement, renders the plasmonic nanocavity an extremely small yet bright nonlinear optical source for the development of next generation chip-scale photonic nanodevices.

© (2018) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Dangyuan Lei, "Nano-optics of single plasmonic particle-on-film nanocavities (Conference Presentation)," Proc. SPIE 10722, Plasmonics: Design, Materials, Fabrication, Characterization, and Applications XVI, 107220V (Presented at SPIE Nanoscience + Engineering: August 21, 2018; Published: 17 September 2018); https://doi.org/10.1117/12.2320157.5836038585001.